In the use of rechargeable batteries, it is well known that discharge of certain types thereof past a certain point results in essentially irreversible chemical changes within the batteries which often result in the inability to fully charge the batteries thereafter and which shorten the useful lives of the batteries. Since rechargeable or secondary batteries are more expensive to purchase than primary or non-rechargeable batteries, schemes have been devised to warn the rechargeable battery user of the state of discharge of the batteries. All known schemes make use of the fact that as the battery is exhausted, the terminal voltage thereof, particularly under load, decreases. Some warning schemes include the use of warning lights which are lit or extinguished when the battery voltage drops below a certain level; others cause the shut-off of the device or portions thereof when the battery voltage drops.
In the use of batteries to power portable lamps or lanterns, the long and almost universal use of non-rechargeable batteries, principally the carbon-zinc or Leclanche cells, has developed habits of use which, when applied to lamps employing rechargeable batteries, are detrimental to the batteries. Specifically, it is customary to attempt to drain every last bit of useable energy out of carbon-zinc batteries, since they will be disposed of after discharge thereof. Manufacturers of portable lamps employing rechargeable batteries normally provide instructions regarding the charging, use, and limitations of discharge of the batteries. However, such instructions are often ignored; and when misuse of the batteries results in failure thereof, the manufacturer is blamed. For this reason, warning devices were devised to notify the user of a low voltage state of the battery before permanent damage could occur. However, the warning devices are often ignored. In order to more positively prevent the misuse of the batteries, shut-off devices were employed. Such shut-off devices are effective in protecting the batteries, but it has been found that battery users are often confused by the automatic shut-off of the device employing the batteries, suspecting that a defect in the device is the cause. In general consumers do not understand or expect an instantaneous cutoff of voltage to the device they are using and have in effect been "trained" through use of non rechargeable batteries that such a cutoff without normal dimming of a bulb (or other sign that the batteries are going "dead") means something defective other than the batteries.